October has often been jinxed by big stock market drops. Big monthly declines occurred in 1929, 1971, 1977, 1978, 1979, 1987 and 2008, according to Stock Trader’s Almanac data. But it’s not all bad. Since 1950, the S&P 500 has averaged a 0.70% gain for investors in the 10th month of the year. It’s produced gains in nearly 60% of the recent Halloween months — 38 of the past 64. Will this month be different? So far, not so good.